1. Field of the Invention
The present invention relates to a toner carrier and an image-forming apparatus (such as copying machines and laser printers) equipped with said toner carrier. The toner carrier supplies a toner to the image-forming body, such as photosensitive body which holds an electrostatic latent image and paper, thereby forming a visible image on the image forming body. More particularly, the present invention relates to a toner carrier, which is durable for a long time and capable of giving high-quality images, and an image-forming apparatus equipped with said toner carrier.
2. Description of the Related Art
Conventional image-forming apparatus such as copying machines and printers, which are based on electrophotography, employs the pressure development method to form visible images. This method is designed to supply a one-component toner to the image-forming body (such as photosensitive body), which holds an electrostatic latent image, so that the toner attaches itself to the latent image, thereby making it visible. (See U.S. Pat. Nos. 3,152,012 and 3,731,146.)
According to the pressure development method, the toner carrier (carrying a toner) is brought into contact with the image-forming body (or photosensitive body) holding an electrostatic latent image such that the toner attaches itself to the latent image on the image-forming body, thereby forming an image. For this reason, the toner carrier should be formed from an electrically conductive elastic material.
As FIG. 2 shows, the pressure development method works in the following way. A toner carrier 1 (or a development roll) is installed between a toner applying roll 5 to supply a toner and an image-forming body 6 (or a photosensitive body) to hold an electrostatic latent image. The toner carrier 1, the image-forming body 6, and the toner applying roll 5 turn in the direction of arrows. Thus, the toner applying roll 5 supplies the toner 7 to the surface of the toner carrier 1. The toner supplied is uniformly spread into a thin layer by the layer forming blade 8. As the toner carrier 1 turns, staying in contact with the image-forming body 6, the toner thin layer transfers from the toner carrier 1 to the latent image on the image-forming body 6. Thus, the latent image becomes visible. Incidentally, there are shown in FIG. 2 a transfer part 9, which transfers a toner image to a recording medium such as paper, and a cleaning station 10, which has a cleaning blade 11 to remove residual toner remaining on the surface of the image forming body 6 after transfer.
During this procedure, the toner carrier 1 should turn while staying in close contact with the image-forming body 6. Consequently, the toner carrier 1 consists of a shaft 2 of good conducting material (such as metal) and an electrically conductive elastic layer 3 formed thereon, as shown in FIG. 1. The layer 3 is formed from an elastic rubber or foam (such as silicone rubber, NBR, EPDM, and polyurethane rubber) incorporated with an electrically conducting agent. The layer 3 is covered with a surface coating layer 4 of resin. The coating layer 4 regulates the degree of charging and sticking of the toner 7 and also regulates friction with the image forming body 6 and the layer forming blade 8. Further, it prevents the photosensitive body from contamination with the elastic material.
Another image-forming method has been proposed as disclosed in Japanese Patent Laid-open No. 116559/1983. This method employs a non-magnetic toner which is spread in thin layer on the surface of the toner carrier (in sleeve form). The toner carrier is arranged near the image-forming body (photosensitive body) without contact. The non-magnetic toner flies to the photosensitive body for development and image forming.
There is another image-forming method proposed. This method employs paper or OHP sheet as the image-forming body. The toner held on the toner carrier is allowed to fly directly to this image-forming body through holes made in a control electrode, so that an image is formed directly on paper or OHP sheet.
The above-mentioned image-forming method that employs a sleeve-like toner carrier or a control electrode also has the toner carrier coated with an electrically conductive elastic layer of resin. This coating layer regulates the degree of charging and sticking of the toner and also regulates friction with the image forming body, the layer forming blade, and the control electrode.
It has been proposed to make the coating layer on the surface of the toner carrier from melamine resin, phenolic resin, alkyd resin, fluorocarbon resin, polyamide resin, or the like. The coating layer of these resins will reduce friction of the toner carrier and improve image quality.
Requirements for image-forming have become very stringent recently as the printing speed increases, the image becomes finer than before, and the monochrome image is replaced by the color one. These present a problem which cannot be solved by the conventional toner carrier. When the toner carrier incorporated into the image-forming apparatus is used for a long period of time, image defects occur as the number of print runs increases. The image defects include fogging in white images, harshness in halftone images, and density variation in black images.